On the Influence of Time‐Varying Flow Velocity on Unsteady Aerodynamics

Wall, Berend G. van der; Leishman, J. Gordon

doi:10.4050/jahs.39.25

Cited by 88 publications

(45 citation statements)

References 22 publications

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“…He used the high-frequency assumption which gives the wake Airfoil in a high amplitude oscillating stream 83 vorticity a harmonic sinusoidal form. This may be questionable because it corresponds to a small σ approximation (van der Wall & Leishman 1994). However, using the velocity profile u s , Greenberg's theory leads to the normalized lift coefficient in (2.3).…”

Section: Theorymentioning

confidence: 99%

“…Based on these promising results, the presented extension of Isaacs' theory in form of the unsteady vortex sheet strength is considered to be correct. Accurate evaluation of the complete chordwise bound vortex sheet strength enables a separation of the total lift into two parts, namely the 'impulsive pressure' lift (also known as the 'apparent mass term') and the 'circulatory' lift (also known as 'Joukowski' lift) as suggested for example by Isaacs (1945) and van der Wall (1992).…”

Section: Separation Of the 'Impulsive Pressure Lift' And The 'Circulamentioning

confidence: 99%

“…Isaacs did not imply any additional assumptions regarding the wake. Thus, his results were considered as mathematically exact (van der Wall 1992). Regarding the lift coefficient in (2.2), the term 0.5σ k cos(ωt) represents the non-circulatory solution.…”

Section: Theorymentioning

confidence: 99%

“…Although Isaacs and Greenberg focused on the ratio between the unsteady lift L(t) and the steady lift L s , the ratio of the non-dimensional lift coefficients is considered as the more appropriate parameter for comparison in this study. Thus, all dynamic effects in this paper are quantified by means of the nondimensional lift coefficient ratio represented in (2.1) which is suggested by van der Wall & Leishman (1994). Thereby, the quasi-steady lift coefficient merely depends on the square of the velocity and the steady lift (Isaacs 1945)…”

Section: Theorymentioning

confidence: 99%

“…Based on Theodorsen's and Isaacs' approaches, Greenberg (1947) developed a simplified solution for the dynamic lift of a flat plate in an oscillating free stream including an oscillating angle of attack and oscillating plunge motion. In 1991, van der Wall provided an extensive review of existing theoretical approaches and extended Isaacs' theory to harmonic plunge motion and unsteady angle of attack variations including arbitrary multiples of the free stream harmonic (van der Wall 1992;van der Wall & Leishman 1994). He concluded that Isaacs' theory is the only exact theory without additional simplifications.…”

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confidence: 99%

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Airfoil in a high amplitude oscillating stream

et al. 2016

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A combined theoretical and experimental investigation was carried out with the objective of evaluating theoretical predictions relating to a two-dimensional airfoil subjected to high amplitude harmonic oscillation of the free stream at constant angle of attack. Current theoretical approaches were reviewed and extended for the purposes of quantifying the bound, unsteady vortex sheet strength along the airfoil chord. This resulted in a closed form solution that is valid for arbitrary reduced frequencies and amplitudes. In the experiments, the bound, unsteady vortex strength of a symmetric 18 % thick airfoil at low angles of attack was measured in a dedicated unsteady wind tunnel at maximum reduced frequencies of 0.1 and at velocity oscillations less than or equal to 50 %. With the boundary layer tripped near the leading edge and mid-chord, the phase and amplitude variations of the lift coefficient corresponded reasonably well with the theory. Near the maximum lift coefficient overshoot, the data exhibited an additional high-frequency oscillation. Comparisons of the measured and predicted vortex sheet indicated the existence of a recirculation bubble upstream of the trailing edge which sheds into the wake and modifies the Kutta condition. Without boundary layer tripping, a mid-chord bubble is present that strengthens during flow deceleration and its shedding produces a dramatically different effect. Instead of a lift coefficient overshoot, as per the theory, the data exhibit a significant undershoot. This undershoot is also accompanied by high-frequency oscillations that are characterized by the bubble shedding. In summary, the location of bubble and its subsequent shedding play decisive roles in the resulting temporal aerodynamic loads.

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Section: Theorymentioning

confidence: 99%

Section: Separation Of the 'Impulsive Pressure Lift' And The 'Circulamentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

mentioning

confidence: 99%

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Airfoil in a high amplitude oscillating stream

et al. 2016

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Numerical study of dynamic stall effects on VR‐12 airfoil with pitch oscillation and accelerated inflow

Zolghadr,

Khoshnevis

2024

Energy Science & Engineering

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This study investigates the effects of positive horizontal acceleration of the freestream velocity on a pitch‐oscillating VR‐12 airfoil using computational fluid dynamics. The shear stress transport k–ω model, coupled with a low‐Reynolds number correction, was employed for Re <105 during dynamic stall. The flow equations were solved in two‐dimensional, incompressible form using the finite volume method. The study examined various parameters, including positive acceleration values of the inflow and the angle of attack of the airfoil, to determine their impact on lift and drag coefficients, as well as the ratio. Additionally, the maximum lift coefficient was analyzed under different inflow and airfoil motion conditions. The results indicate that aerodynamic force coefficients and the ratio are influenced by both the attack angle and the acceleration of the inflow. Furthermore, inflow acceleration affects the onset of dynamic stall conditions. Generally, inflow acceleration modifies the lift coefficient of the airfoil during the upstroke, while having minimal effect on the drag coefficient, except near dynamic stall points. The findings also suggest that, for a specific airfoil, the sequence of factors with the greatest influence on lift force generation before static stall occurs is as follows: asymmetric airfoil oscillation, symmetrical airfoil oscillation, accelerated inflow, constant velocity inflow, and static airfoil.

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Large eddy simulations of a utility‐scale horizontal axis wind turbine including unsteady aerodynamics and fluid‐structure interaction modelling

2022

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Growing horizontal axis wind turbines are increasingly exposed to significant sources of unsteadiness, such as tower shadowing, yawed or waked conditions and environmental effects. Due to increased dimensions, the use of steady tabulated airfoil coefficients to determine the airloads along long blades can be questioned in those numerical fluid models that do not have the sufficient resolution to solve explicitly and dynamically the flow close to the blade. Various models exist to describe unsteady aerodynamics (UA). However, they have been mainly implemented in engineering models, which lack the complete capability of describing the unsteady and multiscale nature of wind energy. To improve the description of the blades' aerodynamic response, a 2D unsteady aerodynamics model is used in this work to estimate the airloads of the actuator line model in our fluid-structure interaction (FSI) solver, based on 3D large eddy simulation. At each section along the actuator lines, a semiempirical Beddoes-Leishman model includes the effects of noncirculatory terms, unsteady trailing edge separation, and dynamic stall in the dynamic evaluation of the airfoils' aerodynamic coefficients. The aeroelastic response of a utility-scale wind turbine under uniform, laminar and turbulent, sheared inflows is examined with oneand two-way FSI coupling between the blades' structural dynamics and local airloads, with and without the enhanced aerodynamics' description. The results show that the

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On the Influence of Time‐Varying Flow Velocity on Unsteady Aerodynamics

Cited by 88 publications

References 22 publications

Airfoil in a high amplitude oscillating stream

Airfoil in a high amplitude oscillating stream

Numerical study of dynamic stall effects on VR‐12 airfoil with pitch oscillation and accelerated inflow

Large eddy simulations of a utility‐scale horizontal axis wind turbine including unsteady aerodynamics and fluid‐structure interaction modelling

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